Windowless H-bridge buck-boost switching converter

ABSTRACT

A “windowless” H-bridge buck-boost switching converter includes a regulation circuit with an error amplifier which produces a ‘comp’ signal, a comparison circuit which compares ‘comp’ with a ‘ramp’ signal, and logic circuitry which receives the comparison circuit output and a mode control signal indicating whether the converter is to operate in buck mode or boost mode and operates the primary or secondary switching elements to produce the desired output voltage in buck or boost mode, respectively. A ‘ramp’ signal generation circuit operates to shift the ‘ramp’ signal up by a voltage Vslp(p−p)+Vhys when transitioning from buck to boost mode, and to shift ‘ramp’ back down by Vslp(p−p)+Vhys when transitioning from boost to buck mode, thereby enabling the converter to operate in buck mode or boost mode only, with no need for an intermediate buck-boost region.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/856,611, filed Apr. 4, 2013, which claims the benefit of provisional patent application No. 61/704,195, filed Sep. 21, 2012, and to provisional patent application No. 61/772,038, filed Mar. 4, 2013, and are incorporated by reference herein.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to switching power converters, and more particularly to buck-boost switching converters.

Description of the Related Art

A switching converter receives an input voltage VIN and produces an output voltage VOUT. One type of switching converter known as a ‘buck-boost’ converter is capable of operating in either a buck mode in which the output voltage produced is less than the input voltage (VOUT<VIN), a boost mode in which VOUT>VIN, or in a buck-boost mode in which VOUT is nearly equal to VIN.

One suitable application of a buck-boost converter is in a battery-powered system, in which input voltage VIN decreases over time; this is illustrated in FIG. 1a . Input voltage VIN might be provided by, for example, a lithium-ion battery, the voltage of which decays from 4.2 v to 2.5 v over time. If the circuitry being powered by the switching converter requires an operating voltage of, for example, 3.3 v, it is beneficial to use a buck-boost converter. When the battery voltage is above 3.3 v by a minimum amount, the converter operates in buck mode. Similarly, when the battery voltage is below 3.3 v by a minimum amount, the converter operates in boost mode. However, if the battery voltage is just slightly above 3.3 v, the converter is typically unable to continue working in buck mode, and must switch to buck-boost mode operation. Similarly, boost mode typically cannot be sustained when the battery voltage is just slightly below 3.3 v, requiring the converter to switch to buck-boost mode operation.

FIG. 1b shows an “H-bridge” circuit that is commonly used as the final stage of a buck-boost power converter. Two switching elements 10, 12 are connected together at a node 14, between VIN and a circuit common point, and two switching elements 16, 18 are connected together at a node 20, between VOUT and the circuit common point. An inductor L is connected between nodes 14 and 20, and a filter capacitor C is typically connected between VOUT and circuit common. In buck mode, switching element 16 is closed and switching elements 10 and 12 are switched on and off in complementary fashion, typically with pulse-width modulated (PWM) signals, to produce a desired VOUT. In boost mode, switching element 10 is closed and switching elements 16 and 18 are switched in complementary fashion to produce the desired VOUT. However, in buck-boost mode, all four switching elements must be switched to produce the desired VOUT.

A buck-boost mode is conventionally used because a direct transition between buck mode and boost mode can cause a discontinuity in the output voltage. There is a drawback to this method of operation, however: due to the need to be switching all four switching elements, efficiency is poor when operating in buck-boost mode.

SUMMARY OF THE INVENTION

A “windowless” H-bridge buck-boost switching converter is presented which addresses the problems discussed above.

The present switching converter employs primary and secondary high and low side switching elements and an inductor arranged in an H-bridge configuration, with the top of the primary high side switching element being the input node for the converter and the top of the secondary high side switching element being the output node for the converter. The converter includes a regulation circuit arranged to operate the primary or secondary high and low side switching elements as needed to produce a desired output voltage at the output node. The regulation circuit includes an error amplifier arranged to produce a ‘comp’ signal which varies with the difference between a voltage proportional to the output voltage and a reference voltage, a first comparison circuit which compares ‘comp’ with a ‘ramp’ signal and toggles an output when ‘ramp’ increases above or falls below ‘comp’. The regulation circuit also includes logic circuitry which receives the output of the comparison circuit and a mode control signal indicating whether the converter is to operate in buck mode or boost mode. When buck mode operation is indicated, the logic circuitry switches the primary high and low side switching elements to produce the desired output voltage, and when boost mode operation is indicated switches the secondary high and low side switching elements to produce the desired output voltage.

The present converter also employs a ramp signal generation circuit which includes a current sensing circuit that provides an output which varies with the current conducted by one of the high side switching elements, slope compensation and/or offset circuitry which produces an output that is summed with the output of the current sensing circuit to produce a voltage ‘rampl’, a fixed voltage Vhys which is referenced to voltage ‘rampl’ to produce a voltage ‘ramph’ given by ‘rampl’+Vhys, and a switching circuit arranged to provide voltage ‘rampl’ to the first comparison circuit as signal ‘ramp’ during buck mode operation and to provide voltage ‘ramph’ to the first comparison circuit as signal ‘ramp’ during boost mode operation.

The output of the slope compensation and/or offset circuitry is a triangle-shaped slope compensation signal having an amplitude of Vslp(p−p) in buck mode, and the peak DC voltage of the slope compensation signal in boost mode.

A second comparison circuit compares ‘comp’ signal with a ‘ramp#’ signal and toggles an output when ‘ramp#’ increases above or falls below ‘comp’. The switching circuit is further arranged to provide voltage ‘rampl’ to the second comparison circuit as signal ‘ramp#’ when the mode control signal indicates boost mode operation and to provide voltage ‘ramph’ to the second comparison circuit as signal ‘ramp#’ when the mode control signal indicates buck mode operation. Second logic circuitry is arranged to receive the output of the second comparison circuit and to provide the mode control signal in response.

The present arrangement operates to shift the ‘ramp’ signal up by Vhys+Vslp(p−p) when transitioning from buck to boost mode, and to shift ‘ramp’ back down by Vhys+Vslp(p−p) when transitioning from boost to buck mode. This enables the converter to operate with only two modes—buck and boost—with no need for an intermediate buck-boost region. As such, the number of mode transitions is reduced, and there are never more than two switching elements being switched—thereby providing improved efficiency. Output voltage discontinuities are also minimized with only two operating modes.

These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a diagram illustrating switching converter operating mode vs. input voltage for a conventional buck-boost switching converter.

FIG. 1b is a schematic diagram of a known H-bridge configuration as is typically used with a buck-boost switching converter.

FIG. 2a is a diagram illustrating switching converter operating mode vs. input voltage for a windowless buck-boost switching converter per the present invention.

FIG. 2b is a schematic diagram of an H-bridge configuration as might be used with a windowless buck-boost switching converter per the present invention.

FIG. 3 is a block/schematic diagram illustrating one possible embodiment of a windowless buck-boost switching converter per the present invention.

FIG. 4 is a timing diagram illustrating a buck mode to boost mode transition for a windowless buck-boost switching converter per the present invention.

FIG. 5 is a timing diagram illustrating a boost mode to buck mode transition for a windowless buck-boost switching converter per the present invention.

FIG. 6 is a block/schematic diagram of a current sensing circuit, slope compensation and offset circuit, and mode change circuit as might be used with a windowless buck-boost switching converter per the present invention.

FIG. 7 is a diagram illustrating the operation of ‘ramp’ and ‘ramp#’ signals associated with a windowless buck-boost switching converter per the present invention.

FIG. 8 is a schematic diagram of one possible embodiment of a logic circuit which produces a mode control signal for a windowless buck-boost switching converter per the present invention.

FIG. 9 is a block/schematic diagram illustrating another possible embodiment of a windowless buck-boost switching converter per the present invention.

FIG. 10 is a block/schematic diagram of a current sensing circuit, slope compensation and offset circuit, and mode change circuit as might be used with the windowless buck-boost switching converter of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

A diagram illustrating switching converter operating mode vs. input voltage for a windowless buck-boost switching converter per the present invention is shown in FIG. 2a . The present converter architecture has only 2 operating modes—buck and boost—which serves to reduce efficiency problems found in conventional buck-boost converters, while also minimizing output discontinuities. The converter uses an H-bridge configuration as shown in FIG. 2b , with two switching elements 30, 32 connected together at a node 34 and between VIN and a circuit common point, and two switching elements 36, 38 connected together at a node 40 between VOUT and the circuit common point. An inductor L1 is connected between nodes 34 and 40, and a filter capacitor C1 is connected between VOUT and circuit common. For the present converter, in buck mode, switching element 36 is closed and switching elements 30 and 32 are switched on and off in complementary fashion, typically with pulse-width modulated (PWM) signals, to produce a desired VOUT. In boost mode, switching element 30 is closed and switching elements 36 and 38 are switched in complementary fashion to produce the desired VOUT. At no time are more than two switching elements being switched, thereby improving converter efficiency. The present converter is preferably realized with a true peak current mode buck regulator, a true peak or emulated current mode boost regulator, and a novel method for transitioning between buck mode and boost mode.

A block/schematic diagram of an exemplary embodiment of the present windowless buck-boost converter is shown in FIG. 3. The converter includes primary high and low side switching elements 30, 32 connected together at a node 34, secondary high and low side switching elements 36, 38 connected together at a node 40, and inductor L1, with the switching elements and inductor arranged in an H-bridge configuration. Here, switching elements 30, 32, 36, 38 are realized with PMOS and NMOS FETs, though other switching devices might also be used. The top of primary high side switching element 30 is the input node for the converter and receives an input voltage VIN, and the top of secondary high side switching element 36 is the converter's output node at which output voltage VOUT is provided. Filter capacitor C1 would typically be connected between VOUT and a circuit common point; VOUT is suitably connected to a load 42.

The converter also includes a regulation circuit arranged to operate the primary or secondary high and low side switching elements to produce a desired output voltage VOUT. The regulation circuit includes an error amplifier 50 (which may include compensation circuitry 52) arranged to produce a ‘comp’ signal which varies with the difference between a voltage VFB proportional to VOUT and a reference voltage Vref, and a first comparison circuit 54 which compares the ‘comp’ signal with a ‘ramp’ signal and toggles an output 56 when the ‘ramp’ signal increases above or falls below the ‘comp’ signal. Logic circuitry 58 receives the output 56 of the comparison circuit and a mode control signal 60 indicating whether the converter is to operate in buck mode or boost mode; the logic circuitry is arranged to operate primary high and low side switching elements 30, 32 to produce the desired VOUT voltage when mode control signal 60 indicates buck mode operation, and to operate secondary high and low side switching elements 36, 38 to produce the desired VOUT voltage when mode control signal 60 indicates boost mode operation. The switching elements are switched on and off during an associated switching cycle, and have associated ‘on’ and ‘off’ times. As used herein, a pair of switching elements are ‘operated’ when they are switched on and off in complementary fashion to regulate VOUT, typically using PWM signals. Note that when in buck mode, switching element 40 is on continuously, to provide a conductive path from inductor L1 to the output node (VOUT); similarly, when in boost mode, switching element 30 is on continuously, to provide a conductive path from inductor L1 to the input node (VIN).

Logic circuit 58 would typically include an SR latch 62, which is set on the rising edge of a periodic clock signal 64 ‘set’. The Q output of latch 62 is provided to logic gates 66 and 68, each of which receive mode control signal 60. In this example, logic gate 66 is a NOR gate and logic gate 68 is an AND gate. If mode control signal 60 is low indicating buck mode operation: when latch 62 is set and Q goes high, NOR gate output 70 goes low, primary high side switching element 30 is switched on and low side switching element 32 is switched off. When the ‘ramp’ signal exceeds the ‘comp’ signal, the output 56 of comparison circuit 54 goes high, resetting latch 62 and causing primary high side switching element 30 to be switched off and low side switching element 32 to be switched on. Switching elements 30 and 32 are switched on and off in this way as needed to regulate VOUT.

With respect to the secondary side in buck mode, the output 72 of AND gate 68 remains low as long as mode control signal 60 is low, such that secondary high side switching element 36 is always on (thereby providing a conductive path from L1 to VOUT) and low side switching element 38 is always off.

If mode control signal 60 is high indicating boost mode operation: when latch 62 is set and Q goes high, AND gate output 70 goes high, secondary high side switching element 36 is switched off and low side switching element 38 is switched on. When the ‘ramp’ signal exceeds the ‘comp’ signal, the output 56 of comparison circuit 54 goes high, resetting latch 62 and causing secondary high side switching element 36 to be switched on and low side switching element 38 to be switched off. Switching elements 36 and 38 are switched on and off in this way as needed to regulate VOUT.

With respect to the primary side in boost mode, the output 70 of NOR gate 66 remains low as long as mode control signal 60 is high, such that primary high side switching element 30 is always on (thereby providing a conductive path from L1 to VIN) and low side switching element 32 is always off.

The converter also includes a ramp signal generation circuit 80, which includes a current sensing circuit 82 that provides an output 84 which varies with the current conducted by one of the high side switching elements (here, primary high side switching element 30), slope compensation and/or boost offset circuitry 86 which produces an output 88 that is summed with the current sensing circuit output at a summing node 90, and a mode change circuit 92 which produces the ‘ramp’ signal and generates mode control signal 60 and a complementary mode control signal 166. For the examples described herein, the converter operates in buck mode when mode control 60 is high, and in boost mode when mode control signal 166 is high; note, however, that the converter could be easily adapted for use with mode control signals of opposite polarities.

Ramp signal generation circuit 80 is arranged to shift the ‘ramp’ signal up when transitioning from buck to boost mode, and to shift ‘ramp’ back down when transitioning from boost to buck mode, which enables the ‘comp’ voltage to remain relatively constant during mode transitions. A transition from buck to boost mode is illustrated in FIG. 4. The diagram includes the periodic ‘set’ signal provided to latch 62, the sensed inductor current IL, the voltages at nodes 34 (SWA) and 40 (SWB), the ‘ramp’ signal and the ‘comp’ signal. The ‘ramp’ signal is the sum of a signal proportional to the inductor current (proportional to IL) and the slope compensation signal (which has an amplitude Vslp(p−p)), and has a slope equal to that of the slope compensation signal.

Assume that the system is in buck mode. At first, the ‘ramp’ signal reaches the ‘comp’ signal during every cycle, causing PWM pulses (SWA) to be delivered to the primary high and low side switching elements. However, if input voltage VIN begins to decrease, output voltage VOUT will decrease as well, causing ‘comp’ to increase. As ‘comp’ increases, the buck mode duty cycle (given by VOUT/VIN) eventually increases to near 100%, the slope of the sensed inductor current IL (proportional to VIN−VOUT) falls to near 0, and the ‘ramp’ signal can no longer reach the ‘comp’ signal; at this point (100), the PWM pulses are suspended (102). If ‘comp’ becomes greater than ‘ramp’ plus a hysteresis voltage Vhys (104), suitably 10-100 mv, the transition to boost mode is made. This is accomplished by shifting the ‘ramp’ signal up by a voltage Voff(boost) which is given by Vslp(p−p)+Vhys. After the transition, the boost mode duty cycle, given by 1−VIN/OUT, is around 0%, and PWM pulses (SWB) are delivered to the secondary high and low side switching elements.

By shifting the ‘ramp’ signal up at the transition point in this way, the ‘ramp’ signal valley comes to just below the ‘comp’ signal, such that the duty cycle does not vary much after the transition. This can minimize output voltage ripple at the mode transition.

A transition from boost to buck mode is illustrated in FIG. 5. Here, a transition is necessitated when the output voltage increases and the ‘comp’ signal decreases. The boost mode duty cycle eventually decreases to near 0%, the slope of the sensed inductor current IL falls to near 0, and the ‘ramp’ signal valley no longer reaches the ‘comp’ signal (106). If ‘comp’ becomes less than ‘ramp’ minus hysteresis voltage Vhys (108), the transition to buck mode is made. This is accomplished by shifting the ‘ramp’ signal down by voltage Voff(boost)=Vslp(p−p)+Vhys. After the transition, the buck mode duty cycle is around 100%.

One way of implementing the present windowless buck-boost converter is to generate two ramp voltages ‘ramph’ and ‘rampl’, with ‘ramph’ being equal to ‘rampl’+Vhys, and then using ‘rampl’ as the ‘ramp’ signal which is compared with the ‘comp’ signal when in buck mode, and using ‘ramph’ as the ‘ramp’ signal when in boost mode. Exemplary embodiments of current sense circuit 82, slope compensation and boost offset circuit 86, and mode change circuit 92 which implement this approach are shown in FIG. 6.

Current sense circuit 82 may be implemented with an amplifier 110 (typically a voltage-to-current (V-to-I) converter), the inputs of which are connected across one of the high side switching elements—here, primary high side switching element 30. Current sense circuit 82 produces an output 112 which varies with the current conducted by primary high side switching element 30; current sense output 112 is coupled to summing node 90.

For the exemplary implementation of slope compensation and boost offset circuit 86 shown in FIG. 6, a fixed current source 120 is connected in series with a capacitor 122, and a switch 124 is connected across capacitor 122 which is closed when the ‘set’ signal is asserted. This arrangement operates to provide a sawtooth waveform at a node 126 that serves as a slope compensation signal 128.

The circuit also suitably includes a capacitor 130 connected to node 126 via a ‘set’-operated switch 132. This arrangement operates as a slope amplitude sample-and-hold, which stores the peak of the slope compensation signal at node 126 on capacitor 130. A switch 134 operates to connect the slope compensation signal to summing node 90 when the converter is operating in buck mode, and to connect the sample-and-hold output to summing node 90 when the converter is operating in boost mode. A V-to-I converter 136 may be connected at its input to switch 134, and used to convert the voltage at its input to a current 137 which is provided to summing node 90.

For some applications, it may be necessary to provide boost slope compensation. One arrangement that could be used for this purpose is circuitry 140, which includes a fixed current source 142 connected in series with a capacitor 144 at a node 146, with a switch 148 that is closed during the converter's ‘off’ time connected across capacitor 144. Node 146 is coupled to summing node 90 via a switch 150 which is closed when in boost mode. A V-to-I converter 152 may be used to convert the voltage at node 146 to a current suitable for delivery to summing node 90.

A resistor 160 is suitably connected between summing node 90 and a circuit common point to convert the summed currents into a voltage; this voltage, which may be buffered with a buffer 162, is voltage ‘rampl’. A fixed voltage Vhys is referenced to voltage ‘rampl’ to produce voltage ‘ramph’, which is given by ‘rampl’+Vhys. A switching circuit 164 is arranged to receive the ‘rampl’ and ‘ramph’ signals, along with mode control signals 60 and 166. Switching circuit 164 provides voltage ‘rampl’ to comparison circuit 54 (pwm_comp) as signal ‘ramp’ when the mode control signals indicate buck mode operation, and provides voltage ‘ramph’ to comparison circuit 54 as signal ‘ramp’ when the mode control signals indicate boost mode operation.

To determine the converter's operating mode, a second comparison circuit 170 (mode_comp) is preferably used, which compares the ‘comp’ signal with a ‘ramp#’ signal and toggles an output 172 when ‘ramp#’ increases above or falls below ‘comp’. Switching circuit 164 is preferably further arranged to provide voltage ‘rampl’ to second comparison circuit 170 as signal ‘ramp#’ when the mode control signals indicate boost mode operation, and to provide voltage ‘ramph’ to the second comparison circuit as signal ‘ramp#’ when the mode control signals indicate buck mode operation. Logic circuitry 174 is arranged to receive the output 170 of the second comparison circuit and to provide mode control signals 166 and 60 in response.

An illustration of the operation of the ‘ramp’ and ‘ramp#’ signals is shown in FIG. 7. Initially, the converter is operating in buck mode; as such, ‘rampl’ is provided to the pwm_comp comparison circuit as signal ‘ramp’, and ‘ramph’ is provided to the mode_comp comparison circuit as signal ‘ramp#’. As long as ‘ramp#’ exceeds ‘comp’ during each cycle, the converter will remain in buck mode. However, if the output 172 of the mode_comp comparison circuit is low (176) at a peak of the ‘ramp#’ signal, this indicates that ‘ramp#’ failed to reach the ‘comp’ signal, and thus the converter should transition to boost mode.

With the converter operating in boost mode, ‘rampl’ is provided to the mode_comp comparison circuit as signal ‘ramp#’, and ‘ramph’ is provided to the pwm_comp comparison circuit as signal ‘ramp’. As long as the valley of ‘ramp#’ falls below ‘comp’ during each cycle, the converter will remain in buck mode. However, if the output 172 of the mode_comp comparison circuit is high (178) at a valley of the ‘ramp#’ signal, this indicates that ‘ramp#’ failed to fall below the ‘comp’ signal, and thus the converter should transition back to buck mode. To summarize, during buck mode, ‘ramp’=‘rampl’ and ‘ramp#’=‘ramph’, and during boost mode, ‘ramp’=‘ramph’ and ‘ramp#’=‘rampl’.

One possible implementation for logic circuit 174 is shown in FIG. 8. D flip-flops 180, 182 each receive the output 172 of the mode_comp comparison circuit at their data inputs, and the ‘set’ signal at their clock inputs. The Q output of flip-flop 182 is gated (184) with mode control signal 60 (boost), with the result 186 provided to the ‘set’ input of an SR latch 188, and the Q output of flip-flop 180 is gated (190) with mode control signal 166 (buck), with the result 192 provided to the ‘reset’ input of the SR latch. In operation, if in buck mode and the output 172 of the mode_comp comparison circuit is low when ‘set’ pulses, the Q output of flip-flop 180 goes high, causing SR latch 188 to reset, such that mode control signal 166 (buck) goes low and mode control signal 60 (boost) goes high—signaling the converter to transition to boost mode. Similarly, if in boost mode and the output 172 of the mode_comp comparison circuit is high when ‘set’ pulses, the Q output of flip-flop 182 goes high, causing SR latch 188 to be set, such that mode control signal 166 (buck) goes high and mode control signal 60 (boost) goes low—signaling the converter to transition to buck mode.

Though the examples above depict the sensed current being that in the primary high side switching element, the present windowless buck-boost converter can also be arranged such that the current in the secondary high side switching element is sensed instead. The transition method between buck and boost remains as described above, but in boost mode the converter employs an emulated peak current mode control scheme. An exemplary implementation of such a converter is shown in FIG. 9. The circuit blocks are the same as in FIG. 3, except that the mode change block 200 now includes circuitry to generate an emulated current signal in boost mode (discussed below), and current sense circuit 82 is arranged to sense the current in secondary high side switching element 36 instead of primary high side switching element 30.

Exemplary embodiments of current sense circuit 82, slope compensation and boost offset circuit 86, and mode change circuit 200 which implement the approach depicted in FIG. 9 are shown in FIG. 10. Current sense circuit 82 is as before, with an amplifier 110 connected across primary or secondary high side switching element 30 or 36, and producing a current 112 which varies with the sensed current. Slope compensation and offset circuit 86 is also as before, providing the slope compensation signal at node 126 to summing node 90 when the converter is operating in buck mode, and the output of sample-and-hold capacitor 130 to summing node 90 when the converter is operating in boost mode, preferably as a current 137.

Mode change circuit 200 is also similar to that of mode change circuit 92, except for the addition of circuitry to generate an emulated current signal in boost mode. In this example, the additional circuit comprises a switch 202 connected between summing node 90 (or, preferably, the output of a buffer 204) and a node 206, a capacitor 208 connected between node 206 and circuit common, a fixed current source 210, and a switch 212 connected between the output of current source 210 and node 206. In operation, if in buck mode, switch 202 is always closed and switch 212 is always open, so that the converter operates as before. If in boost mode, switch 202 is closed during the converter's ‘off’ time and open during the converter's ‘on’ time, and switch 212 is closed during the converter's ‘on’ time and open during the converter's ‘off’ time, such that an emulated current is summed with the signal at summing node 90 in boost mode.

The converter can be arranged such that, in buck mode, the current in either the primary or secondary high side switching element can be sensed, with regulation provided with a true peak current mode control scheme. In boost mode, if the current in the primary high side switching element is sensed, regulation can be either true or emulated peak current mode. If the current in the secondary high side switching element is sensed, boost mode regulation must be accomplished with emulated peak current mode.

The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims. 

The invention claimed is:
 1. A method for operating a switching converter having pairs of high and low side switching elements to provide buck and boost operating modes, the method comprising: providing a ramp signal having a slope that is based on (1) a current sense signal that varies with a current in a switching element of the converter and on (2) a specified compensation signal; providing an error signal that is based on a difference between a specified reference signal amplitude and an output signal amplitude from the switching converter; and when the error signal transitions to be greater than a sum of the ramp signal and a specified voltage magnitude, increasing the ramp signal by an offset amount that is at least a sum of the specified voltage magnitude and a peak-to-peak amplitude of the ramp signal and is sufficient to transition the converter from buck mode to boost mode without transitioning through an intermediate buck-boost mode; and when the error signal transitions to be less than a difference of the ramp signal and the specified voltage magnitude, decreasing the ramp signal by an offset amount that is at least a sum of the specified voltage magnitude and a peak-to-peak amplitude of the ramp signal and is sufficient to transition the converter from boost mode to buck mode.
 2. The method of claim 1, further comprising: providing a mode control signal based on the current sense signal and the specified compensation signal; and updating an on/off status of the pairs of high and low side switching elements in the switching converter using a logic circuit, the logic circuit including a first input configured to receive a result of a comparison of the ramp signal and the error signal, a second input configured to receive the mode control signal, and a third input configured to receive a clock signal, and the logic circuit including an output coupled to the switching elements.
 3. The method of claim 1, further comprising generating the specified compensation signal at a first terminal of an energy storage element, including: charging the energy storage element using a fixed current source that is coupled with the energy storage element, and selectively discharging the energy storage element to provide a substantially sawtooth signal waveform.
 4. The method of claim 3, further comprising updating an on/off status of at least one of the high side switching elements and at least one of the low side switching elements to operate the switching converter in a buck mode or in a boost mode according to a duty cycle signal and a periodic clock signal; wherein the selectively discharging the energy storage element includes using a switch that is triggered by the periodic clock signal.
 5. The method of claim 1, further comprising, in the buck operating mode, providing the specified compensation signal as a time-varying sawtooth waveform signal, and in the boost operating mode, providing the specified compensation signal as a DC signal having an amplitude that corresponds to a peak-to-peak amplitude of the time-varying sawtooth waveform signal.
 6. The method of claim 1, further comprising transitioning from buck mode to boost mode, including operating the switching elements according to a first duty cycle adjacently preceding the transition, the first duty cycle being substantially about 0% or 100% duty cycle, the transitioning further including operating the switching elements according to a second duty cycle adjacently following the transition, the second duty cycle being substantially the other of 0% or 100% duty cycle.
 7. The method of claim 1, further comprising sensing the current sense signal using a sensing amplifier circuit that is coupled to source and drain terminals of one of the high side switching elements.
 8. The method of claim 1, wherein the providing the ramp signal includes providing a signal that is a sum of the current sense signal, the specified compensation signal, and a boost compensation signal, wherein the boost compensation signal is provided during ‘off’ portions of the converter's cycle.
 9. A buck-boost power switching converter system with primary and secondary switching elements arranged in an H-bridge configuration with an inductor, at least one of the switching elements corresponding to an input node, and at least one of the switching elements corresponding to an output node, and the system comprising: a ramp signal generator circuit configured to generate a ramp signal having a slope that is based on (1) a current sense signal that varies with a current in a switching element of the converter and on (2) a specified compensation signal; and a regulation circuit configured to selectively operate the primary and secondary switching elements in a selected one of a buck converter mode or in a boost converter mode, wherein the regulation circuit includes a comparator circuit to provide an error signal based on a difference between a specified reference signal amplitude and an output signal amplitude from the switching converter system; when the error signal transitions to be greater than a sum of the ramp signal and a specified voltage magnitude, the ramp signal generator circuit configured to increase the ramp signal by an offset amount that is at least a sum of the specified voltage magnitude and a peak-to-peak amplitude of the ramp signal and is sufficient to transition the converter from buck mode to boost mode without transitioning through an intermediate buck-boost mode; and when the error signal transitions to be less than a difference of the ramp signal and the specified voltage magnitude, the ramp signal generator circuit configured to decrease the ramp signal by an offset amount that is at least a sum of the specified voltage magnitude and a peak-to-peak amplitude of the ramp signal and is sufficient to transition the converter from boost mode to buck mode.
 10. The buck-boost power converter system of claim 9, wherein the ramp signal generator circuit includes: a current sense circuit that provides an output that varies with an inductor current of the inductor; a slope compensation and/or boost offset circuit that provides one of a time-varying sawtooth signal or a DC signal; a summing node that receives the output from the current sense circuit and at least one of the time-varying sawtooth signal and the DC signal; and a mode change circuit that receives a signal from the summing node and, in response, provides the ramp signal based on the output from the current sense circuit and based on at least one of the time-varying sawtooth signal and the DC signal from the slope compensation and/or boost offset circuit.
 11. The buck-boost power converter system of claim 10, wherein the ramp signal generator circuit includes a switching circuit, and the switching circuit selectively provides one of two different ramp signals based on an indication of a buck or boost mode, wherein the two different ramp signals are offset from each other by a specified voltage signal amplitude.
 12. The buck-boost power converter system of claim 10, wherein the current sense circuit includes a voltage-to-current converter circuit having inputs coupled across one of the primary or secondary switching elements to sense information about a current in the element.
 13. The buck-boost power converter system of claim 10, wherein the slope compensation and/or boost offset circuit includes: a fixed current source; an energy storage element coupled in series with the fixed current source; and a switch configured to selectively discharge the energy storage element according to a set signal; wherein the set signal is selected to provide a substantially sawtooth compensation signal at the summing node for use in the buck converter mode.
 14. The buck-boost power converter system of claim 13, wherein the slope compensation and/or boost offset circuit includes a sample-and-hold circuit coupled to the energy storage element and configured to store peak information about the substantially sawtooth compensation signal.
 15. The buck-boost power converter system of claim 13, further comprising a boost compensation circuit for use in boost mode, the boost compensation circuit having an output coupled to the summing node and the boost compensation circuit including: a second fixed current source; a second energy storage element coupled in series with the second fixed current source; and a second switch configured to selectively discharge the second energy storage element during the converter's off time.
 16. The buck-boost power converter system of claim 9, wherein the ramp signal generator circuit generates a mode control signal that indicates whether the system is to operate in buck mode or boost mode, and wherein the mode control signal is based on the information received from the one or more of the primary and secondary switching elements, and is based on a specified slope compensation signal.
 17. The buck-boost power converter system of claim 16, wherein the regulation circuit includes: a logic circuit having first and second inputs, wherein the first input receives the mode control signal from the ramp signal generator circuit; and a comparison circuit that provides a reset signal to the second input of the logic circuit, wherein the reset signal is based on a comparison of the ramp signal and the error signal; wherein the logic circuit issues control signals to the primary and secondary switching elements based on the mode control signal and the reset signal.
 18. The buck-boost power converter system of claim 9, further comprising: the primary switching elements including primary high and low side switching elements that are coupled in series at a first node; the secondary switching elements including secondary high and low side switching elements that are coupled in series at a second node; and the inductor coupled between the first and second nodes.
 19. The buck-boost power converter system of claim 18, further comprising: a PWM comparator circuit that compares the ramp signal with the error signal; and a logic circuit that includes an SR latch and logic gates, wherein the logic gates are configured to selectively switch the primary and secondary switching elements according to an output signal from the SR latch, and wherein the SR latch includes a clock input and a data input, and wherein the data input is configured to receive a result of the comparison of the ramp signal and the error signal from the PWM comparator circuit. 